The invention is directed to a process for acceleration of the solidification of hydraulic cement mixtures by addition of crystalline zeolites and to the compositions obtained.
Solidification accelerators lead to a definite acceleration of the solidification of cement mixtures from hydraulic binders and aggregates of various types. The accelerating effect of the additive is, e.g., particularly in demand with sprayed concrete, in the sealing of water breaks and in the employment of anchor iron.
Additives of this type for cement bonded building materials accordingly are of economical and industrial interest.
It is already known to use calcium chloride as an additive for the accelerated solidification of hydraulic cements. However, this additive leads to stronger "creeps" of concrete and in dangerous manner promotes the corrosion of metal reinforcement. Therefore, the employment of calcium chloride as well as the other chlorides is forbidden in Germany.
Furthermore, it is known to use other calcium salts such as calcium thiosulfate (K. Murakami et al Yogyo Kyokai Shi 1968, 76, 373-383, Chem. Abst. 70 (1969) 90486), calcium sulfamate (German OS No. 2053308), and calcium nitrite (French Pat. No. 1498369) in hydraulically hardening cement mixtures. Other organic and inorganic materials, because of their limited effectiveness, have until now only been of moderate interest.
Calcium formate has also been proposed repeatedly as setting and solidification accelerator for hydraulic cement mixtures (German OS No. 1963375 and German AS No. 1232861). However, because this material, above all at too high dosage, causes too quick a solidification of the cement mixtures, and because its accelerating effect is confined to the early phase of the setting process, the practical use of calcium formate has remained limited.
Calcium thiocyanate likewise is available for its accelerating effect with hydraulic cement mixtures. In comparison to calcium formate, there is even less danger of too quick solidification. Besides, with calcium thiocyanate there is also attained a noticeable acceleration of hardening. However, calcium thiocyanate is not suited for a wide use in cement mixtures because its strong hydroscopicity impairs its ability to be handled and consequently excludes mixing into dry cement. Furthermore, at high amounts of addition there is the danger of corrosion of the concrete reinforcements. Not the least is the fact that with extensive use it is very expensive because of the high amount added.
Furthermore, it is known to add water insoluble activated silicas or silica containing materials to hydraulically hardening cement mixtures to produce water and oil permeable concrete structural parts. Thereby, there has not been detected an accelerated solidification (German Pat. No. 1646437).
Trialuminum phosphate have an Al.sub.2 O.sub.3 :P.sub.2 O.sub.5 ratio of 1:1 in lower amounts of addition up to about 1 percent acts as a setting retarder, while at higher amounts it acts as an accelerator. The narrow limits of activity require an exact maintenance of the prescribed amounts. However, in practice this frequently leads to difficulties.
Therefore, it was the problem of the invention to find an additive which has good accelerating effect but not the disadvantages of the previously mentioned materials.